Saturday, April 13, 2013

I ran into the sources for today's post while searching for the answer to a great question posed by +Bruce Elliott regarding yesterday's post[1] about flux pinning and stable superconductor levitation. Bruce's question was:

"...Of course, like any good post, it leads to more questions. So here's my next one: I would think that since the the induced currents resist any change to the magnetic field, that they would have an effect analogous to friction, i.e. any attempt to change the position of the conductor (or magnet) would be similar to moving an object through a very viscous fluid - hard to do, but once done, the object stays put in the new position.
What we see in the NASA video, however, is that when the levitating magnet is poked, it oscillates slightly. This looks more like the motion of an object in a potential well, like a mass on a spring. This suggests that the superconductor "remembers" the equilibrium position (and the corresponding magnetic field flux) and that the induced currents continually direct the magnet back to that position. Why would the currents not maintain the new position, like a rock being pushed through mud?..."

I don't have a solid answer yet, because I got sidetracked looking for some of the source material for the answer. Before I take us down the sidetrack though, let me explain very briefly the answers I'm trying to track down regarding flux pinning. First, pinning and pinning strength vary with the construction of the superconductor. If a ceramic YBCO superconductor is made from rather loosely packed material, or if there are a lot of defects, then there are lots of sites in the material where magnetic field can penetrate and get pinned. The superconductor in the video that Bruce mentions[2]

was very large and very porous, so it's easy to 'pin' a magnet to it. Flux pinning sites are easily formed. On the other hand, if the same type of superconductor is made from a single crystal domain, then there are far fewer pinning sites due to defects, and it's actually very difficult to levitate a magnet over it unless the magnet is placed there before the superconductor is cooled, allowing the magnetic flux lines to penetrate while the material is not in its superconducting state. You can see this kind of behavior in the following reserach video from Cornell.

The difficulty with which magnetic field can penetrate the superconductor is overcome by suspending the magnet above the superconductor while it is being cooled.

Two more points and I'll get to the sidetrack of the day. The first point is that suspended magnets can move quite easily as long as the magnetic field in the path they move is constant. This leads to things like the train track demo shown in yesterday's post. The second point is that there is a potential well, much like Bruce mentioned, formed as a result of pinning. Cornell researcher +Joseph Shoer discusses both of these things in an excellent poster presentation.

Now, for the sidetrack. While looking for more videos of Cornell's supercondcutor research, I came across a paper about several spacecraft flyby anomalies that have been recorded since 1990[5][8]. Basically, while tracking various spacecraft including the Galileo and Cassini missions it was discovered that the resultant speed of the spacecraft after passing by the Earth to modify its orbit, was a few mm/s faster or slower than was expected. These aren't huge differences mind you since the spacecraft speed was on the order of km/s. None the less, there's not a solidly known reason as to why this is happening yet. One commentator from the European Space Agency has offered some perspective as to the cause of the anomoly in Nature's news column circa 2008[6]:

"Trevor Morley, an engineer at the European Space Agency's (ESA) operations centre in Darmstadt, Germany, notes that the trajectories of spacecraft flybys are calculated using detailed models that must incorporate a lot of phenomena, including the pull of other planets, relativistic effects (changes in time and length for objects travelling near the speed of light), and even the radiation pressure of sunlight striking the craft. One of these may be incorrectly accounted for, or there may be an effect missing from the equations entirely."

That hasn't stopped a number of articles from being written studying the data and hyptohesizing that the cause might be anythign from Lorentz forces acting on electrically charged spacecraft, to dark matter to gravitational anomalies [7][9]. The last referenced paper is from Dr. Nieto who I mentioned a few weeks ago in reference to coherent states, quantum mechanical phase operators and the Pioneer anomaly[10][11].